The objective of this research is to demonstrate that the Ia-like molecule is the target of a human lymphokine (CIL) which shows inhibitory activity on progenitor cells of the hemopoietic system and on Ia+ cells of the immune response. This 85-kilodalton protein factor, which is present in the supernatant of a clone (MT1) of our human T-cell hybrid, will be purified and tested: (1)\in sensitive animals to determine its physiological activity; (2)\in vitro, against bone marrow cells from patients with Ia+ leukemia; and (3)\in nude mice to determine whether it has protective activity against the growth of human or mouse Ia+ tumors. (HF)